This invention relates generally to a sensor system for remote detection and imaging of objects in a backscattering medium such as water. More particularly, this invention relates to a method and apparatus for detecting, locating and/or imaging underwater objects in shallow water and in coastal regions from an airborne platform using a novel imaging lidar (light detection and ranging) system which improves imaging in such shallow water areas.
It is desirable in a number of military and civilian applications to search a volume within a backscattering medium for the presence of certain targets. For instance, moored or bottom mines deployed in ocean shipping lanes are a hazard to navigating ships used both for military and for commercial purposes. For other civilian applications such as law enforcement on the ocean, it is desirable to detect the presence of submerged fishing nets or drug-carrying containers used in smuggling contraband. In or near harbors and beaches, it is also desirable to detect submerged obstructions, cables, pipelines, barrels, oil drums, etc. In strictly military applications, anti-submarine warfare demands an effective means of detecting and locating submarines.
Presently, cumbersome and time consuming wire line devices must be used for detecting underwater targets from remote airborne locations. These devices are lowered into the water and are easily subject to damage and loss. Also, wire line devices make target searching relatively slow and can only detect targets without providing visual imaging.
An improved and novel system for remote detection and imaging of objects underwater (or objects obscured by other backscattering media which are at least partially transmitting to light such as ice, snow, fog, dust and smoke) from an airborne platform has been described in U.S. Pat. No. 4,862,257 and U.S. Pat. No. 5,013,917, both of which are assigned to the assignee hereof and incorporated herein by reference. The imaging lidar system of U.S. Pat. No. 4,862,257 utilizes a laser to generate short pulses of light with pulse widths on the order of nanoseconds. The laser light is expanded by optics and projected down toward the surface of the water and to an object or target. U.S. Pat. No. 5,013,917 relates to an imaging lidar system intended for night vision.
Imaging lidar systems of the type described hereinabove are also disclosed in commonly assigned U.S. Pat. No. 4,964,721 and U.S. Pat. No. 4,967,270, both of which are incorporated herein by reference. U.S. Pat. No. 4,964,721 relates to an imaging lidar system which controls camera gating based on input from the aircraft onboard altimeter and uses a computer to thereby adjust total time delay so as to automatically track changing platform altitude. U.S. Pat. No. 4,967,270 relates to a lidar system employing a plurality of gated cameras which are individually triggered after preselected time delays to obtain multiple subimages laterally across a target image. These multiple subimages are then put together in a mosaic in a computer to provide a complete image of a target plane preferably using only a single light pulse.
Still other imaging lidar systems are disclosed in commonly assigned U.S. Pat. Nos. 5,029,009 and 5,034,810, both of which are incorporated herein by reference. U.S. Pat. No. 5,029,009 describes an imaging lidar system incorporating an imaging camera having a plurality of gating electrodes on a focal plane and means for producing variable time delay gating across an image viewed by the focal plane. U.S. Pat. No. 5,034,810 relates to a two wavelength lidar imaging system for underwater application utilizing image subtraction to detect passage of internal waves or other anomalies under water.
U.S. Ser. No. 565,631 filed Aug. 10, 1990 which is also assigned to the assignee hereof and fully incorporated herein by reference, relates to an airborne imaging lidar system which employs a plurality of pulsed laser transmitters, a plurality of gated and intensified array camera receivers, an optical scanner for increased field of regard, and a computer for system control, automatic target detection and display generation. U.S. Ser. No. 565,631 provides a means for rapidly searching a large volume of the backscattering medium (e.g., water) for specified targets and improves upon prior art devices in performance as a result of having more energy in each laser pulse (due to simultaneous operation of multiple lasers) and a more sensitive detection system using multiple cameras. The several cameras may be utilized to image different range gates on a single laser pulse or several cameras can be gated on at the same time to provide independent pictures which can then be averaged to reduce the noise level and improve sensitivity. Both of these improvements result in higher signal-to-noise ratio and thus higher probability of detection or greater range of depth capability.
Imaging lidar systems of the type hereinabove discussed are often used in conjunction with computerized automatic target detection (ATD) systems for detecting, locating and identifying targets from a plurality of two dimensional images. Examples of such ATD systems are described in commonly assigned U.S. application Ser. Nos. 565,425 and 565,424, both of which were filed on Aug. 10, 1990 and are incorporated herein by reference.
While the imaging lidar systems described above are well suited for their intended purposes (particularly deep ocean detection), there continues to be a need for imaging lidar systems of this type which have improved operational performance and efficiency in the imaging of underwater targets found in shallow water and/or coastal regions. Such coastal zone surveillance presents an even more challenging problem than deep ocean detection. These difficult problems are encountered as a result of the more challenging conditions in shallow water than those encountered previously in deeper water. These factors include highly variable conditions at the air/water interface such as breaking waves, foam and spray, spatially varying bottom depths both parallel and perpendicular to the coastline and the need for high spatial resolution in order to detect and classify small objects which may be partly hidden in the sand.
Notwithstanding the foregoing, the need for imaging lidar systems, suitable for such coastal and shallow water applications is of great interest to government and commercial activities. For example, there continues to be an increasing need for identification of natural and man made obstacles and hazards in the coastal regions. Obvious applications include identifying favorable sites for construction and utilization of shallow water ports and moorings, surveillance of coastal areas to monitor changes in the surf zone as a result of sediment loading or storm activity, and localization and identification of underwater debris as part of search and rescue operations. Other applications involve the detection of obstacles, mines and hidden explosive charges in shallow waters, offshore and in certain riverine environments. Natural and man made underwater effluent and biological fouling of underwater intake and discharge lines such as that caused by the Zebra mussel in the Great Lakes also needs to be identified.
An imaging lidar apparatus specifically configured to meet the problems associated with imaging objects in shallow water and surf zones is disclosed in U.S. application Ser. No. 774,665 filed Oct. 11, 1991 (which is assigned to the assignee hereof and incorporated herein by reference). This apparatus comprises an airborne imaging lidar system for detection and classification of objects in the surf zone of coastal waters. Preferably, the lidar transmitter is a repetitively pulsed Nd:YAG laser illuminating a portion of the coastal marine area for imaging by a gated camera (e.g., CCD camera) with optimized gating time delay. This prior apparatus also utilizes high spatial resolution and target detection algorithms to classify objects and discriminate against clutter. As a result, this device is capable of rapid search of coastal areas, performs real time automatic target detection, noise rejection and classification, and is capable of both day and night operations under a wide range of sea state and water clarity.
Important features of the apparatus of U.S. Ser. No. 774,663 include multiple imaging cameras to provide high spatial resolution, a bottom tracker to maintain camera gating at the optimum local bottom depth, a specialized beam projection system to minimize brightness variations across the field of view and which optimizes the spatial relationship of the transmitted light to the geometry of the receiver optics, and finally, target detection computers to provide real time detection and classification of surf zone obstacles.
The apparatus of U.S. Ser. No. 774,663 finds significant utility in mine detection. Mines and other obstacles laid in the ocean surf zone are frequently used to deter amphibious assault forces. These mines and obstacles can be laid in place on the beach or on the bottom in shallow water, or can be moored above the bottom in relatively shallow water. Using electro-opical, stand-off detection of the type disclosed in U.S. Ser. No. 774,663, such minefields can be used to counter such coastal defenses by determining the locations and densities of surf zone mines. Thus, mine-free zones may be selected for assualt or, at the very least, relatively low density areas may be selected which will result in minimum casualties.
Notwithstanding these important features and advantages, there continues to be a need for imaging lidar systems of the type described in U.S. Ser. No. 774,663 having even more improved spatial resolution and target locating abilities than is currently available from prior art systems. This need is particularly apparent in the field of mine detection. It is desirable to fly the sensor platform parallel to the water line and somewhat offshore. It is also desirable to search the beach, surf zone, and shallow water zone in a single pass. This is not practical to do with the staring sensor of U.S. Ser. No. 774,663 because of the very large number of pixels required to effectively detect mines as small as six inches in diameter.
The range gate isotemporal contours are spheres, and this means that large instantaneous field of view (IFOV) cameras associated with the system of U.S. Ser. No. 774,663 are not desirable since, in this case, part of the image may be the air above the water, part may be water surface glints, part may be water volume backscatter, part may be ocean bottom reflections, and Part may even be "below" the bottom, thus containing only residual sunlight which leaks through the narrow bandpass filter in the camera which is matched to the laser wavelength. Thus, for two reasons, pixel density and range gate curvature, the camera IFOV must be small.